Method and device for dispensing a fluid from a pressure tank

ABSTRACT

To insure in a simple and economical manner a uniform and continuous dispensing of a fluid from a pressure tank, a method is provided according to which a pressurized gas is introduced into the pressure tank via a proportional valve that is disposed in an inlet line of the pressure tank, the pressure of the fluid located in an outlet line is measured with a first pressure sensor, and an outlet valve in the outlet line is opened and closed. The method also includes the determination of a set pressure value as a function of the measurement result of the first pressure sensor, the transfer thereof to the proportional valve, the measurement of the gas pressure in the inlet line with a second pressure sensor disposed between the proportional valve of the pressure tank, and the transfer of the measurement result to the proportional valve. The invention also provides an apparatus for carrying out the above method.

BACKGROUND OF THE INVENTION

The present invention relates to a method for dispensing a fluid from apressure tank, whereby the introduction of a pressurized gas into thepressure tank is provided via a proportional valve that is disposed inan inlet line of the pressure tank, the measuring of the pressure of thefluid located in the outlet line is provided by a first pressure sensor,and the opening and closing of an outlet valve in the outlet line isprovided for. The invention further relates to an apparatus fordispensing a fluid from a pressure tank, and includes a proportionalvalve that is disposed in an inlet line of the pressure tank forintroducing a pressurized gas, a first pressure sensor in an outlet lineof the pressure tank for measuring the pressure of the fluid located inthe outlet line, and an outlet valve in the outlet line.

Such methods and apparatus are known, for example, in coating systemsfor the manufacture of CDs. With these systems, it is important for auniform and continuous coating of the CDs that the pressure of thelacquer provided at the outlet or dispense valve preferably has aconstant predetermined value. With the known systems, the pressure valuemeasured at the first pressure sensor is utilized as an actual value foran adjustment of the proportional valve. However, this results in theproblem that the pressure measured at the first sensor drops due todynamic line and filter pressure losses as soon as the outlet valve isopened. Due to this drop in pressure, the pressure at the proportionalvalve is readjusted until the actual value again coincides with theprescribed desired value. In this connection, readjustment also takesplace during a dispensing process, which leads to imprecision withregard to the applied quantity of lacquer. After the closing of theoutlet valve, the dynamic line and filter pressure losses no longer haveany effect, and the measured pressure at the first pressure sensor againrises to an increased value. The proportional valve must again bereadjusted since the previously introduced gas, which is generallynitrogen, is vented; the readjustment takes place until the actual valueagain corresponds to the desired value.

These control or adjustment processes lead to fluctuations of the dosingvolume as well as to a high consumption of nitrogen. Furthermore,concentration changes and possibly a crystallization of the lacquerdissolved in the solvent can result due to solvent evaporation and adischarge of the solvent vapor together with the nitrogen that escapesduring the readjustment. In addition, continuous oscillations of theregulator occur during a tank filling state with critical residence gasvolumes, which leads to a greater consumption of nitrogen.

It is therefore an object of the present invention to provide a methodand apparatus for dispensing a fluid from a pressure tank, according towhich a uniform and continuous dispensing of the fluid is ensured in asimple and economical manner.

SUMMARY OF THE INVENTION

The stated object is inventively realized with a method of theaforementioned type in that a desired or set pressure value isdetermined as a function of the measurement result of the first pressuresensor and is transferred to the proportional valve, and the gaspressure in the inlet line is measured with a second pressure sensordisposed between the proportional valve and the pressure tank and istransferred to the proportional valve. By the measurement of the gaspressure in the inlet line, and the transfer of the measurement resultsto the proportional valve, the frequent readjustment of the proportionalvalve described above during opening and closing of the outlet valve isavoided, since no dynamic line and filter pressure losses occur betweenthe proportional valve and the second pressure sensor. This leads to alow consumption of nitrogen since no discharge of nitrogen occurs duringclosing of the outlet valve, as a result of which also a lower change ofthe dye concentration in the solvent is achieved. Furthermore,continuous oscillations during the regulation process are suppressed,since the control loop formed by the proportional valve in the secondpressure sensor is not oscillatory. As a consequence of thedetermination of a set pressure value as a function of the measurementresult of the first pressure sensor, and transfer of this value to theproportional valve, there is effected an automatic adaptation of thesystem to changeable disturbance variables, such as tank filling stateand filter pressure losses, as a result of which a stable regulatingcondition is achieved without oscillations and a high dosing precisionis also achieved.

Pursuant to one preferred specific embodiment of the invention, for thedetermination of the set pressure value only those measurement resultsof the first pressure sensor are used that were measured with the outletvalve opened in order to prevent pressure changes, which occur when theoutlet valve is opened or closed, from influencing the determination ofthe desired or set value. For a constant dosing volume flow, only thepressure at the external sensor with the valve opened is relevant and ofinterest. In this connection, only those measurement results of thefirst pressure sensor are used that were measured after a specificperiod of time after the opening of the outlet valve in order thatoscillations that occur during the opening will have no influence uponthe determination. In order to achieve a uniform dispensing of thefluid, for the determination of the set pressure value a measurementresult of the first pressure sensor determined over a measurementinterval is used.

The set pressure value, in addition to being determined as a function ofthe measurement result of the first pressure sensor, is preferablydetermined as a function of the measurement result of the secondpressure sensor in order to achieve a better uniformity and suppressionof disturbance variables. A pressure difference between the inlet andoutlet line is preferably measured, whereby in one specific embodimentof the invention, the set pressure value is determined as a function ofthe measured pressure differential.

Pursuant to a particularly preferred specific embodiment of theinvention, the determination and/or transfer of the set pressure valueis carried out only when the outlet valve is closed to order to ensurethat during a dispensing process no change of the prescribed setpressure value, and a readjustment possibly connected therewith, occur.

Pursuant to a further specific embodiment of the invention, the fillingstate height of the pressure tank is determined as a function of themeasurement results of the first and second pressure sensors in order toprovide an automatic indication thereof and to be able to correct thethereby resulting disturbance variables during the determination of theset pressure value.

For an automatic indication of a filter state, the state of a filterlocated in the outlet line is preferably determined as a function of themeasurement results of the first pressure sensor. From the automaticindication it can be determined when a filter change is necessary. Inthis connection, the filter state is preferably determined as a functionof a difference of the measurement results of the first pressure sensorwith the outlet valve closed and opened. During the determination of thefilter state, preferably only those measurement results are used thatwere measured after conclusion of a predetermined period of time afterthe closing or after the opening of the outlet valve in order thatoscillations that result during the closing or opening do not have aninfluence upon the determination.

The object of the present invention is realized with an apparatus of theaforementioned type in that a control unit is provided for thedetermination, as a function of the measurement result of the firstpressure sensor, of a set pressure value that is to be provided to theproportional valve, and a second pressure sensor is provided between theproportional valve and the pressure tank for measuring the gas pressurein the inlet line and for transferring the measurement result to theproportional valve. With such an apparatus, the advantages describedabove in reference to the method are achieved. For a particularly simpleand economical embodiment of the invention, the second pressure sensoris preferably integrated in the proportional valve.

Pursuant to a further advantageous specific embodiment of the invention,a differential pressure sensor is provided and is disposed between theinlet and the outlet line.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be subsequently explained with the aid of onepreferred specific embodiment with reference to the drawings; in thedrawings:

FIG. 1 shows an inventive apparatus for dispensing a fluid;

FIG. 2 is a flow diagram that illustrates the automatic determination ofa set pressure value;

FIG. 3 is a flow diagram that illustrates the determination of a filterstate and a volume flow; and

FIG. 4 is a graph showing dead and measurement times of pressuresensors.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a coating system 1 for CDs 2.

The coating system 1 has a pressure tank 4 with an inlet line 5 and anoutlet line 6. The inlet line 5 is connected to an upper side of thepressure tank and communicates with an upper region of the pressure tank4. The outlet line 6 is similarly connected to an upper side of thepressure tank 4. However, the outlet line 6 extends through the interiorof the pressure tank 4 and communicates with the interior of thepressure tank in a lower region thereof. The pressure tank 4 ispartially filled with a lacquer 8 for coating the CDs 2. Due to theweight of the lacquer 8, it;completely fills a lower portion of thepressure tank 4. Disposed in the region located above the lacquer is apressurized gas 10, such as nitrogen.

The nitrogen 10 is fed into the pressure tank 4 from a nitrogen supply12 that is connected to the inlet line 5. Disposed in the inlet line 5between the nitrogen supply 12 and the pressure tank 4 is a valve unit14 having a proportional valve 15 and an internal pressure sensor 16that is disposed between the proportional valve 15 and the pressure tank4.

Provided adjacent to the upper side of the pressure tank 4 is adifferential pressure sensor 18 having a first sensor element 19 that isdisposed in the inlet line 5, and a second sensor element 20 that isdisposed in the outlet line 6. Above the sensor element 20, in theoutlet line 6, are furthermore disposed a filter 22, a pressure sensor24, as well as an outlet valve 26. Downstream of the valve 26 (viewedfrom the pressure tank 4) the outlet line 6 opens to a coating station28 in which CDs 2 are supplied with the lacquer 8 when the valve 26 isopened.

The valve unit 14, the differential pressure sensor 18, the pressuresensor 24, as well as the valve 26 are respectively connected to acontrol unit 30. The control unit 30 is provided with an input portion31 that receives the measurement results of the internal pressure sensor16, of the differential pressure sensor 18, and of the pressure sensor24. An output portion 32 of the control unit 30 is connected to thevalve 26 in order to control the opening and closing of the valve 26.The output portion 32 is furthermore connected to the proportional valve15 of the valve unit 14 in order to prescribe for the proportional valve15 a set pressure value calculated by the control unit 30. An output ofthe internal pressure sensor 16 is also connected to the proportionalvalve 15 in order to provide an actual pressure value to theproportional valve 15 in the inlet line 5. The proportional valve 15 ofthe valve unit 14 is in the position, due to the prescribed set pressurevalue from the control unit 30, as well as the actual pressure valuefrom the internal pressure sensor 16, to adjust the pressure in theinlet line 5 to the set pressure valve. Thus, a control loop is formedwithin the valve unit 14 into which a set pressure value is externallyintroduced by the control unit 30.

During operation of the coating system 1, the pressure tank 4 is broughtto a prescribed pressure level by introducing the pressurized nitrogen.Due to the pressure that exists in the pressure tank, the lacquer 8 thatis in the pressure tank is pressed upwardly in the outlet line 6 in thedirection of the valve 26. When the valve 26 is closed, the system isessentially static, and no flow of lacquer takes place. If a CD 2 isdisposed in a coating position in the coating station 28, the valve 26,as controlled by the control unit 30, is opened for a specific period oftime that is necessary for applying a coat of lacquer on the substrate2. With the valve 26 opened, due to the pressure that exists in thepressure tank 4, lacquer 8 flows through the outlet line 6 to thecoating station 28. After the valve 26 is closed, a static systems againresults in which no lacquer flows.

So that in successive coating steps a uniform quantity of lacquer isapplied to the substrate 2, the pressure in the system, especially inthe vicinity of the outlet 26, must be kept at a constant value when theoutlet valve 26 is opened. This is achieved by keeping the pressure inthe outlet line 6, especially in a region downstream of the filter 22(seen from the pressure tank 4) at a constant level. For this purpose, aset pressure value is prescribed for the valve unit 14 for regulatingthe pressure in the system, which set pressure value is related to thepressure measured by the pressure sensor 24.

The calculation of the set pressure value will be explained subsequentlywith the aid of the flow diagram of FIG. 2.

In a first block 40, the pressure P_(ext) is measured at the firstpressure sensor 24. This measured value is conveyed further to adecision block 42 that establishes whether the measured values arederived from a period of time in which the outlet valve 26 is opened orclosed.

If the measurement results are derived from a period of time in whichthe outlet valve 26 is closed, then the measurement results are conveyedfurther to a block 44, which from the measurement results calculates anaverage pressure value P_(ext, OFF, average).

If the measurement results are derived from a period of time in whichthe valve 26 is opened, the measurement results are conveyed further toa block 46 that calculates an average pressure valueP_(ext, ON, average).

After the calculation of the average pressure value P_(ext, ON average),this value is conveyed further to a block 48. In the block 50, as afunction of the desired volume stream of the lacquer to the coatingstation, a desired pressure value P_(desired) is prescribed. Thispressure value P_(desired) is conveyed further to the block 48.

In a block 52, a pressure P_(int) is measured at the pressure sensor 16of the valve unit 14. The measurement results are conveyed further to ablock 54, in which an average pressure value P_(int, average) iscalculated therefrom. This average pressure value is conveyed further tothe block 48.

In the block 48, a roughly approximated or corrected set pressure valueP_(desired, corrected, rough) is calculated as a function of thepressure values introduced in the block 48. The roughly approximated setpressure value results from the following equation:

P _(desired, corrected, rough) =P _(desired)+(P_(int, average) −P_(ext, average))

This value is conveyed further to a block 56, in which an average valueP_(desired, corrected, rough, average) is calculated, which in turn isconveyed further to a block 58.

In a block 60, the pressure differential ΔP_(tank) between the inletline and the outlet line is measured and is conveyed further to a block62, where the measurement results are averaged. The measured pressuredifferential ΔP_(tank) is related to the fill height of the lacquer 8 inthe pressure tank 4, which is calculated therefrom. The averagedpressure differential ΔP_(tank), average is transferred the block 58. Ina block 64, a tolerance range ΔP_(tolerance) for successive measurementsis determined and transferred to the block 58.

In the block 58 it is determined whether the last roughly approximated,averaged set pressure value P_(desired, corrected, rough, average)obtained in the block 56 relative to a previously obtained value lieswithin the tolerance range. This is determined as follows:

((P _(desired, corrected, rough, average, j+1) −P_(desired, corrected, rough, average, j))²)^(0.5) >ΔP _(tolerance),

where j+1 indicates the last calculated average, and j indicates thepreviously calculated average value.

When the above relationship is fulfilled, then the following applies

P _(desired, corrected, j+1) +P_(desired, corrected, rough, average, j+1)

i.e. the newly approximated set pressure value corresponds to theaverage value determined in Block 56.

If the above relationship is not fulfilled, then the newly approximatedset pressure value is determined with the aid of the following equation:

P _(desired, corrected, j+1) =P _(desired, corrected, j)+(ΔP_(tank, average, j+1) −ΔP _(tank, average, j)).

The regulation or adjustment with the aid of the measurement results ofthe differential pressure sensor 18 is effected on the basis of agreater measurement precision of the differential pressure sensor 18relative to the pressure sensor 24, as a result of which the frequencyof “rough” readjustment procedures and in particular the amplitude ofthe readjustment changes are reduced. Consequently, less nitrogen isused.

The new, approximated set pressure value P_(desired, corrected) thatresults from the determination in the Block 58 is subsequently conveyedfurther to the Block 66. In the Block 66, it is established whether thecoating system is within a start phase, i.e. for example within thefirst five coating cycles. If the system is not within a start phase,then the set pressure value determined in the Block 58 is conveyed viathe Block 68 to a Block 70 in order there to form the new, approximatedset pressure value P_(desired, corrected, new).

However, if the system is within a start phase, in a Block 72 an adaptedor an approximated start set pressure valueP_(desired, corrected, start) is determined, among others, with the aidof the characteristics of the lacquer, the geometrical relationships ofthe system, the pressure difference at the differential pressure sensor,and the volume flow of the lacquer.

V_(start) can be determined on the basis of the geometricalrelationships, the pressure relationships in the outlet line, as well asthe characteristics of the lacquer, whereby due to the lack ofmeasurement results one proceeds on the basis that the pressure at thepressure sensor is P_(ext)=P_(desired). If the filter is new, the filterconstant K_(filter, start) corresponds to the manufacturingspecifications. If the filter is used, it corresponds to the lastdetermined and stored value. Alternatively, it is also possible todetermine the filter constant in that prior to the actual coating of aCD, a test output cycle is carried out in which the filter constant isthen determined.

This start value determination is necessary since when the system isinitiated P_(ext, ON) is not known. The set value correction provides aroughly approximated start value.

The approximated start set pressure value is subsequently conveyedfurther to the Block 70, where it forms the new, approximated setpressure value P_(desired, corrected, new).

This value is conveyed further to the Block 74, in which it isdetermined whether the system is presently in a static or dynamic state.If the system is in a static state, i.e. if no medium flows, then thenew, approximated set pressure value P_(desired, corrected, new) istransferred via a Block 76 to a Block 78. If it is determined in theBlock 74 that the system is in a dynamic state, i.e. a medium flows,then the new, approximated set pressure valueP_(desired, corrected, new) is not conveyed further to the Block 78, butrather the previous value found in the Block 78 is retained.

In the Block 78, the approximated set pressure valueP_(desired, corrected, new) is converted into units utilizable for theproportional valve 15 and, for presetting a set pressure value, istransmitted to the valve as P_(valve) in the Block 80.

The determination of the filter state will be explained subsequentlywith the aid of the flow diagram of FIG. 3.

To determine the filter state, the pressure P_(ext) must also beevaluated with the outlet or dispense valve 26 closed. The differencebetween the pressure P_(ext, open) measured at the pressure sensor 24with the outlet valve opened and P_(ext, closed) with the outlet valve26 closed resulted from the filter pressure loss and the line pressureloss in the conduit in conformity with the following equation:

P _(ext, closed) −P _(ext, opened) =ΔP _(filter) +ΔP _(conduit,)

whereby with a constant tank pressure and volume flow the pressure lossin the conduit is also constant. However, over a longer period of timethe filter pressure loss can increase, even at constant tank pressure,if the filter becomes clogged. Consequently, the approximated setpressure value P_(desired, corrected, new) must be increased to the sameextent.

If in so doing a threshold value is reached, an alarm message appearsthat indicates that the filter is used up and must be exchanged.

Furthermore, the filter state should be indicated during the operation.The calculation of the filter state will be described with the aid ofthe flow diagram of FIG. 3.

In a Block 90, the pressure P_(ext) is measured in the outlet line 6 atthe pressure sensor 24. The measured pressure values are conveyedfurther to a Block 92 in which it is determined whether the outlet ordispense valve 26 is opened or closed. If the outlet valve 26 is closed,the measured pressure values P_(ext) are transferred to a Block 94 thatcalculates an average value of the measured pressure values with thevalve 26 closed of P_(ext, OFF, average).

If it is determined in Block 92 that the outlet valve 26 is opened, thenthe measured pressure values P_(ext) are conveyed to a Block 96 thatcalculates an average value P_(ext, ON, average) from the pressurevalues P_(ext) measured with the valve open.

Subsequently, in a Block 98 the volume flow of the lacquer, and in Block100 the pressure loss in the outlet line ΔP_(conduit) are determined.The average value P_(ext, OFF, average), the average valueP_(ext, ON, average) as well as the pressure loss in the lineΔP_(conduit) are transferred to a Block 102 in which a pressure loss inthe filter ΔP_(filter) is calculated. The pressure loss in the filter isdetermined with the aid of the following equation:

ΔP _(filter) =ΔP _(ext, OFF, average) −ΔP _(ext, ON, average) −ΔP_(conduit.)

Subsequently, in a Block 104; an average value ΔP_(filter, average) ofthe pressure loss at the filter is calculated.

Subsequently, in a Block 106, a filter constant K_(filter) is calculatedwith the aid of the determined filter pressure loss and with the aid ofthe volume flow. This is done with the aid of the following equation:

K _(filter) =ΔP _(filter, average) ·A _(filter)/(V·η_(medium))

This value is transferred to a Block 108 in which is determined if thesystem is in a start phase.

If the system is in a start phase, then it is determined in a furtherdecision Block 110 if previously a filter change took place or not. If afilter change took place prior to the new start, then in a Block 112 forthe above calculation of the start set pressure valueP_(desired, corrected, start) the filter constant K_(filter, start) isspecified the same as the filter constants of a new filterK_(filter, new). This value K_(filter, new) is determined from themanufacturing specifications in the Block 114 and is transferred to theBlock 112.

If it is established in the Block 110 that no filter change took placeprior to the new start, then in a Block 116, for the starting valuedetermination, the filter value constant K_(filter, start) is specifiedthe same as the last determined and stored, prior to the shutdown of thesystem, value K_(filter, observed).

If it is determined in the Block 108 that the system is not in a startphase, the filter constant K_(filter) calculated in the Block 106 isconveyed via a Block 118 to the Block 120 as an instantaneous filterstate factor K_(filter, instantaneous). In the Block 120, the filterstate factor is calculated by the following equation:

Filter state factor=K _(filter, instantaneous) /K _(filter, new).

The filter state factor indicates by how much the instantaneous filterpressure loss is greater than the original. In this way, it can bebetter estimated how long the old filter can still be used before afilter change is necessary.

FIG. 4 shows the pressures measured at the internal pressure sensor 16and at the pressure sensor 24 over an opening cycle of the valve 26. Theupper curve shows the pressures measured at the internal pressure sensor16, while the lower curve, which varies relatively significantly, showsthe pressures measured at the pressure sensor 24.

As can be seen from FIG. 4, the pressures measured at the internalpressure sensor 16 are independent of whether the outlet valve 26 isclosed or opened, and are relatively constant.

However, the pressures measured at the pressure sensor 24 varyrelatively significantly during opening or closing of the outlet valve26. In this connection, directly after the closing there results arelatively significant drop in pressure, which subsequently againincreases in order to then again drop. Thus, in a time interval T1 afterthe opening of the outlet valve 26 relatively significant fluctuationsof the pressure result. After the time interval T1, there is a phasehaving a relatively constant pressure. After the closing of the outletvalve 26, there is a relatively significant increase in pressure, whichsubsequently again drops and rises. Thus, in a time interval T2 afterthe closing of the valve relatively significant fluctuations result.After the time interval T2, there is again a phase having a relativelyconstant pressure.

For the above calculation of a corrected set pressure value, as well asfor the calculation of the filter state factor, therefore exclusivelymeasurement results are used that originate from a phase in which thepressure fluctuations resulting from the opening and closing of thevalve have essentially subsided. These phases are indicated in FIG. 4 asmeasurement T1 for measurements with the valve opened, and measurementT2 for measurements with the valve closed.

Although the apparatus has been described with the aid of a preferredspecific embodiment assuming a coating system for CDs, the apparatus isnot limited thereto.

The specification incorporates by reference the disclosure of Germanpriority document 199 14 203.3 filed Mar. 29, 1999, German prioritydocument 199 37 606.9 filed Aug. 9, 1999 and International prioritydocument PCT/EP00/02155 of Mar. 11, 2000.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

What is claimed is:
 1. A Method of dispensing a fluid from a pressuretank, including the steps of: introducing a pressurized gas into saidpressure tank via a proportional valve that is disposed in an inlet lineof said pressure tank; measuring the pressure of fluid in an outlet lineof said pressure tank via a first pressure sensor; determining a setpressure value as a function of a measurement result of said firstpressure sensor; transferring said set pressure value to saidproportional valve; measuring a gas pressure in said inlet line via asecond pressure sensor that is disposed between said proportional valveand said pressure tank, and transferring a resulting measurement resultto said proportional valve; and opening and closing an outlet valve insaid outlet line.
 2. A method according to claim 1, wherein saiddetermination of said set pressure value, only those measurement resultsof said first pressure sensor are used that were measured when saidoutlet valve was open.
 3. A method according to claim 2, wherein onlythose measurement results of said first pressure sensor were used thatwere measured after a specific period of time after opening of theoutlet valve.
 4. A method according to claim 1, wherein saiddetermination of said set pressure value, a measurement result of saidfirst pressure sensor determined over a measurement interval is used. 5.A method according to claim 1, wherein said set pressure value is alsodetermined as a function of a measurement result of said second pressuresensor.
 6. A method according to claim 1, wherein a pressuredifferential is measured between said inlet line and said outlet line.7. A method according to claim 6, wherein said set pressure value isalso determined as a function of said pressure differential.
 8. A methodaccording to claim 1, wherein a filling state height of said pressuretank is determined as a function of measurement results of adifferential pressure sensor.
 9. A method according to claim 1, whereinat least one of said determination and transfer of said set pressurevalue is effected only when said outlet valve is closed.
 10. A methodaccording to claim 1, wherein a filling state height of said pressuretank is determined as a function of measurement results of said firstand second pressure sensors.
 11. A method according to claim 1, whereina state of a filter disposed in said outlet line is determined as afunction of measurement results of said first pressure sensor.
 12. Amethod according to claim 11, wherein said filter state is determined asa function of a difference of the measurement results with the outletvalve closed and with the outlet valve opened.
 13. A method according toclaim 11, wherein during said determination of said filter state, onlythose measurement results are used that were measured after theconclusion of a predetermined period of time after closing or opening ofsaid outlet valve.
 14. An apparatus for dispensing a fluid from apressure tank, comprising: a proportional valve that is disposed in aninlet line of said pressure tank and serves for introducing apressurized gas into said pressure tank; a first pressure sensordisposed in an outlet line of said pressure tank for measuring thepressure of fluid located in said outlet line; an outlet valve in saidoutlet line; a control unit for determining, as a function of ameasurement result of said first pressure sensor, a set pressure valuethat is to be provided said proportional valve; and a second pressuresensor, disposed between said proportional valve and said pressure tank,for measuring a gas pressure in said inlet line and for transferring aresulting measurement result to said proportional valve.
 15. Anapparatus according to claim 14, wherein said second pressure sensor isintegrated in said proportional valve.
 16. An apparatus according toclaim 14, wherein a differential pressure sensor is disposed betweensaid inlet line and said outlet line.